1. Field of the Invention
The present invention relates generally to an automatic automotive transmission and more specifically to a control system for such a transmission which features tri-stage shift shock attenuating line pressure control during shifting operations.
2. Description of the Prior Art
In automotive transmissions it is conventional when a transmission upshifts (for example), for the friction element or elements which are involved in producing the speed from which the shift is being made, to be released before the one or ones which produce the speed to which the shift is being made, are engaged. During the engagement of the latter mentioned elements the level of the line pressure is controlled solely with respect to the load on the engine (viz., the position of the accelerator pedal or throttle valve).
However, this results in the drawback that, as shown in FIG. 9, during the initial and final stages of the friction element engagement relatively large shift shock inducing fluctuations in the torque transmission (see spikes indicated by P and Q), occur.
In order to obviate this problem the arrangement disclosed in JP.A.60 231059 was proposed. With this arrangement the level of the line pressure is controlled to a single essentially constant level from the time the shift actually begins to take place until the time it actually finishes. However, even with this control the spikes P and Q and the resulting shift shock are not satisfactorily suppressed.